This invention relates to a nuclear fuel element and a method for fabricating the nuclear fuel element, and more particularly to a nuclear fuel element and the method for fabricating same, in which is charged a material (such as for instance boron carbide) that causes a (n, .alpha.) reaction, i.e., the reaction whereby the aforesaid material absorbs neutrons and then produces helium. (Such a reaction is referred to as (n, .alpha.) reaction.)
At the present time, the nuclear element for use in a boiling water reactor has a plurality of UO.sub.2 pellets charged in its fuel cladding, both ends of which are sealed with end plugs. The UO.sub.2 pellets are produced by molding power of uranium dioxide, sintering same and grinding the uranium dioxide thus sintered. When the UO.sub.2 pellets are charged into the fuel cladding, then there will be defined a narrow gap between the UO.sub.2 pellets and the fuel cladding. This gap is provided so as to prevent the adverse effects on the fuel cladding due to the deformation of UO.sub.2 pellets caused by thermal expansion or swelling. Helium maintained at about 1 Atm. is charged into the fuel cladding beforehand. A fuel assembly is composed of a plurality of nuclear fuel elements, and then the fuel assembly is placed in the core of a reactor pressure vessel. Helium maintained at about 1 Atm. is charged in a gap which is defined between the fuel cladding and the UO.sub.2 pellets charged into the initial nuclear fuel element placed in the core of the reactor pressure vessel, while only helium is present in the gap. However, when the boiling water reactor is put into operation and then the operation is continued for a certain period of time, then there are discharged from UO.sub.2 pellets various kinds of fission products in the fuel cladding. Included in the fission products are krypton, xenon and the like. Gases (principally, moisture, carbon dioxide gas, nitrogen and the like) which have been absorbed to the surfaces of the UO.sub.2 pellets, as well, are discharged into the fuel cladding. The various kinds of gases which have been discharged from the UO.sub.2 pellets are accumulated in a gas plenum provided in the upper portion of the fuel cladding and in a gap defined between the UO.sub.2 pellets and the fuel cladding. Krypton and xenon are extremely low in their thermal conductivity, as compared with that of helium. (For instance, at 450.degree. C., the thermal conductivity of krypton is about 6.8% of that of helium, and that of xenon is about 4.1% of that of helium.) Krypton and xenon, which have been discharged from UO.sub.2 pellets with lapse of the operating time of the boiling water reactor, are accumulated in the fuel cladding gradually, and thus the absolute amount thereof is gradually increased. As a result, the ratio of the gases having extremely low thermal conductivity, such as krypton and xenon, which occupy the gap defined between the UO.sub.2 pellets and the fuel cladding, is increased. As the quantities of krypton, xenon and the like are increased, the thermal conductivity between the UO.sub.2 pellets and the fuel cladding is lowered. (This thermal conductivity will be referred to as "gap conductance", hereinafter.) It has been confirmed that the UO.sub.2 pellets within the nuclear fuel element placed in the core of the boiling water reactor shrink during the operation of the boiling water reactor, i.e., the density of the UO.sub.2 pellets is increased to a high level. Upon the occurrence of such phenomenon, the gap defined between the UO.sub.2 pellets and the fuel cladding is somewhat increased during the operation of the reactor. Accordingly, the gap conductance is further lowered and hence there results insufficient cooling of UO.sub.2 pellets, while the temperature of the overall UO.sub.2 pellets is raised. If the temperature of UO.sub.2 pellets is too high, because of the failure of piping of the re-circulating system and the resulting loss of coolant, then there will be increased a danger of the failure of the fuel cladding before the emergency cooling device has been operated. Upon the failure of fuel cladding, fission products and nuclear fuel having strong radioactivity and placed in the nuclear fuel element leak into the cooling water, and thus there will be increased a danger of the fission products and nuclear fuel leaking from the failed points in the piping to outside the reactor pressure vessel or the reactor containment.